A vacuum conveyor transports material powder, granulate, dust, tablets, small parts etc, in a suction gas flow, as a rule, air or inert gas.
Material is taken in from a feed location, e.g., with a hand-held suction line, feed funnel or similar, transported through a hose or pipe, and reaches the vacuum conveyor by means of an intake opening. In the vacuum conveyor, an inserted filter element, aided by centrifugal force and sedimentation, separates the transported material from the suction gas. The transported material is collected in the separating vessel of the vacuum conveyor. The filtered suction gas flow is diverted from the separating vessel to flow through the vacuum pump driving the conveying process. Said vacuum pump commonly is installed in a stationary manner directly on the upper part of the vacuum conveyor or in the immediate vicinity thereof and is to be protected—even independently of protecting the environment—from contamination. After the process of filling the separating vessel has been carried out, the vacuum pump is switched off or the suction gas flow between the separating vessel and vacuum pump is interrupted by means of a valve mounted on the filtered clean side or gas side
For a standard vacuum conveyor, a discharge valve mounted at the bottom of the separating vessel now opens and the transported material falls out of the separating vessel through the discharge opening
Bridging materials can be fluidized as an aid, or pushed out of the separating vessel with overpressure. The filter is cleaned of adhering filter residue by means of a counter blow from the clean gas side. The duration of individual intake and discharge cycles commonly is controlled by means of a sequence control system having adjustable intake and discharge times. Intake and discharge times for a vacuum conveyor commonly are relatively short and, as a rule, are only several seconds each. Since filter residue can be cleaned very frequently, this brief cycle time permits an extremely compact structure having extremely small filtering surfaces in comparison to the inflow velocity in a conventional air-filter installation.
If solid matter is conveyed in the suction gas flow, then a separation of electrical charge can occur due to contact of individual material particles with one another and also due to contact with the conveyor lines. In this connection, the material and also the contacted surfaces receive an electrostatic charge, resulting in the risk of ignition by means of various types of electric discharge for ignitable solid matter or present combustible gases as well as mixtures thereof. Said effect can be counteracted in a vacuum conveyor by means of suitable grounding measures. Gaining acceptance in particular are solutions featuring a completely, constantly electrically conductive separating vessel. Hose assemblies should be equipped with wire spirals grounded at both ends connected to the separating vessel via metal, for example, a clamp. In the conveyor itself, all electrically conductive parts are connected to one another in an electrically conductive manner and are grounded, so the risk of a spark discharge in the conveyor can be minimized and essentially ruled out.
However, conveyed material itself retains a charge that can be dissipated only slowly by means of the vessel wall of the separator or by means of atmospheric moisture. Also, a risk of explosion originates from the filter residue adhering to the filter of the separator—with sufficiently great quantities of charge, the surface charge can lead to so-called brush discharges.
According to current science, brush discharges for solid matter having a minimum ignition energy MIE greater than 1 mJ are not capable of ignition, such that a vacuum conveyor for solid matter having an MIE greater than 1 mJ generally can be utilized. However, for safety reasons, the quantity of matter per conveying cycle is limited to a mass of approximately 10 kg in order to prevent larger build-ups of charge.
However, if combustible gases or gases which support a combustion process, such as oxygen, are present in the surroundings, even lower energy levels are sufficient to ignite said gases. In said cases, inertization regularly is selected as a measure of explosion protection. In this connection, the oxygen content in particular is reduced to a non-critical level for the respective application and replaced by means of a suitable inert fluid. Examples include N2, CO2, or noble gases.
For an inertization process for vacuum conveyors known from the state of the art according to EP 0 937 004 B1, material is taken from a feed point into the separating vessel of a conveyor, the intake opening of the vacuum conveyor is closed and the separating vessel of the conveyor is evacuated to a sufficiently high vacuum. The vacuum pump device is then isolated from the separating vessel by means of a valve and the separating vessel is rendered inert from the clean gas side of the filter.
In order to ensure that a nearly complete gas exchange has occurred and the oxygen content thus reduced to a non-critical level, an inertization process occurring three times in succession has gained acceptance among known methods. This means that after the intake of material has taken place, the separating vessel is evacuated three times and rendered inert in order to ensure a gas exchange above the separated material, in particular. The material is subsequently discharged under a nitrogen blanketing. A disadvantage of the previous method therefore lies in the fact that in order to ensure non-critical oxygen content in the gas or fluid present in the separating vessel, time-intensive multiple inertizations must take place.
It now has been established that the inert fluid fed into a separating vessel above the separated material has little interaction with the volume of separated material and the risk exists that oxygen present in the gap volume and/or pore volume of the separated material is not exchanged at all or is only incompletely exchanged with the inert fluid with the aid of the aforementioned inertization process.